In application Ser. No. 367,883 filed June 7, 1973 by Mansson et al., now U.S. Pat. No. 3,854,085 non-destructive testing apparatus of the magnetic flux leakage type is disclosed in which a plurality of electromagnetic pickups or probes simultaneously scan different portions of an object passing thereby. Each probe produces a flaw signal having positive and negative excursions, and is preferably a core of magnetic material having spaced legs with a coil wound around the core. The probe outputs are amplified and the positive and negative portions separately rectified and combined to yield positive and negative signals corresponding to the largest signal of each polarity simultaneously occurring in the probes. These largest signals are added to yield bipolar signals having corresponding positive and negative excursions. The bipolar signals are then supplied to one or more processing channels for producing flaw indications.
In one embodiment a probe assembly having a plurality of longitudinally-spaced probes is mounted on a rotating head through which an object to be tested is passed, e.g. a tube or rod. A magnet on the head produces a transverse magnetic field across the object. Flaws in the object produce leakage flux which is sensed by one or more probes to produce a flaw indication. Two probe assemblies may be diametrically spaced on the rotating head to permit greater speed of travel of the object through the head.
In the specific embodiments of the aforesaid application the probes in a given assembly are in alignment. This has been found to give excellent results in detecting flaws in longitudinal weld seams in tubes, for example, with a high signal-to-noise ratio. Also, spiral defects can be detected for either direction of spiralling, although the signal amplitude may be smaller since a smaller portion of a probe will be effective at a given instant. Other types of flaws can also be detected.
Although capable of giving excellent results under appropriate operating conditions, it has been found that a long shallow longitudinal flaw may give an output signal having an amplitude equal or greater than that for a short deep flaw. In many cases short deep flaws may be more serious than long shallow flaws.
A primary object of the present invention is to provide flux-leakage test apparatus of the foregoing type in which the flaw signal amplitude is largely dependent on the depth of the flaw and independent of its length. Thus the signal amplitude corresponds more closely to the severity of the defect.